As a process for producing hydrogen fluoride (HF) on an industrial scale, a process is conventionally used in which a metal fluoride is reacted with sulfuric acid; oleum and water; or a mixture thereof. The most well known one of such processes is, for example, as follows. Firstly, sulfuric acid (H2SO4) and fluorite (containing CaF2 as its main component) which is the metal fluoride are separately supplied as raw materials to a prereactor (or a kneading machine) such as an external heating type ko-kneader which is capable of solid-liquid kneading. The reaction partially proceeds in the prereactor by kneading and heating the raw materials, so that hydrogen fluoride is generated through the reaction. Next, a reaction mixture thus obtained is transferred from the prereactor to an external heating type rotary kiln. The reaction further proceeds in the external heating type rotary kiln by rotating and heating the reaction mixture, so that hydrogen fluoride is additionally generated. The generated hydrogen fluoride is recovered as a gaseous product. Such prereactor and rotary kiln are generally made of a corrosion resistant alloy consisting of Ni, Cr, Mo, Fe and so on, such as Hastelloy. Alternatively, these are lined with the corrosion resistant alloy. An example of this kind of process for producing hydrogen fluoride is described in Japanese Patent Kohyo publication No. 4-40282.